Piston rings have recently become used in increasingly severer environment, due to higher power of engines, higher combustion temperatures and larger load for meeting exhaust gas regulations, the use of low-viscosity lubricating oils, the diversification of fuels such as bioethanol, higher fuel injection pressure, etc. Even piston rings having hard chromium nitride (CrN) coatings formed by ion plating, which are now considered as having best scuffing resistance and wear resistance, may fail to exhibit sufficient performance due to the peeling of coatings by cracking and chipping. Accordingly, improvement in scuffing resistance, wear resistance and peeling resistance has been more strongly demanded than even.
Because the chromium nitride formed by ion plating generally suffers chipping despite hardness, various improvements such as the controlling of crystal orientations, structures and porosity, the addition of a third element, the lamination of coatings, etc., have been made so far.
With respect to the lamination of coatings, for example, Patent Reference 1 discloses a coating obtained by alternately laminating a columnar layer of CrN crystals oriented in a direction from a substrate surface to a coating surface, and a flat layer, or a coating obtained by alternately laminating a layer having porosity of 0-0.5% by volume and a layer having porosity of 1.5-20% by volume, to prevent the peeling of the coating due to chipping caused by chipping fatigue on an outer peripheral surface. For the same purpose, Patent Reference 2 discloses a thick, hard coating with reduced internal stress and high adhesion, which alternately comprises hard composite nitride layers of columnar crystals (for example, CrSiN and TiSiN), and non-columnar crystal layers relaxing the stress of the hard composite nitride layers, by alternately repeating high bias voltage conditions for forming columnar crystals and low bias voltage conditions for forming non-columnar crystals with constant intervals.
Patent References 1 and 2 disclose laminate coatings of nitrides having different structures (columnar structure and non-columnar structure, or porous structure and dense structure), and Patent Reference 3 discloses a different-composition laminate coating comprising metal layers as the stress-relaxing layers in Patent Reference 2. Though the metal layers relax stress more than the composite nitride layers of Patent Reference 2, they have an adverse effect on scuffing resistance.
Patent Reference 4 discloses a piston ring having a laminate coating formed by alternately laminating compound layers of titanium, carbon and nitrogen, and compound layers of chromium, carbon and nitrogen. To form a coating exhibiting excellent wear resistance without chipping when used for top rings of diesel engines, Patent Reference 4 determines the amount of carbon dissolved in a solid solution and porosity, such that the hard coating has fracture toughness of about 3 MPa·m1/2 or more, and hardness Hv of 1700 or more.
However, the above laminate coatings are not satisfactory for some piston rings used in extremely severe environment, failing to avoid peeling due to cracking and chipping by the fatigue of the hard coatings.